Marine seismic exploration is generally conducted by towing a marine seismic streamer cable at a given depth through the ocean or other body of water. However, in some locations due to congestion on the surface or a requirement to detect shear waves, a marine seismic bottom cable is used.
The current practice in marine seismic data collection is to require marine seismic cable of a longer length than was required in the past. For streamer cable, this means a length which could exceed four miles and for bottom cable the length could be equally as long and with an operating depth in excess of 4000 feet. In addition, some seismic data collection techniques require a plurality of marine streamer cable being towed behind a ship at the same time.
A marine cable includes sections of marine cable, usually about 300 feet long, connected together by couplers. In some marine cable designs, each 600 feet, an instrumentation package is connected between sections of marine cable. Each section and each instrumentation package, if it is to be interchangeable, must be able to absorb the cumulative drag force upon the cable when towed in water and if a bottom cable, it must also be able to absorb the weight of the cable between the ship and the bottom.
A marine cable section can absorb this tension force by including longitudinally within the cable section stress members which connect to the couplers to transfer the drag and weight forces on that cable section to the next section of marine cable until finally at the connection to the ship, the drag force on the cable is received as a cumulative tension force on the cable.
In the case of the instrumentation package connected between two couplers, the drag and weight forces are transferred from the trailing coupler, which is linked to stress members of the trailing section of seismic cable, through the body of the instrumentation package to the connecting coupler of the next section of marine cable.
Longer cable or cable which can operate at greater depths place new demands on marine cable operations. The additional length increases the time on station because additional time is needed to deploy the cable and in some cases, currently available methods to deploy cable may not be adequate to prevent damage to the longer cable.
In the present methods for marine cable deployment the speed at which cable can be deployed into the water without damaging the cable is a limiting factor. Marine cable which meets the new requirements for additional length or greater operating depth place even more sever limitations on the speed at which the marine cable can be deployed.
The problem is the tension on the cable as the cable is wound off of the cable storage reel. This can be explained using the deployment of a streamer cable. A streamer cable, especially if it is being used with other equally long cables, is deployed during towing to keep the cables separated. However, the towing does exert a drag force on the cable which is proportional to the length of the cable. This force is seen at the cable reel as a longitudinal tension force on the cable.
Streamer cable sections can be as much as 4 inches in diameter. They are usually made with an outer layer of polyurethane tube which is supported by spacers and the tube is filled with oil or some other nonconducting liquid to provide buoyancy to the marine cable.
The streamer cable is fragile and flexible at the surface. Current designs of marine cable can usually withstand the longitudinal tension force on the cable at the point on the cable storage reel where the cable is winding off of the reel. However, the longitudinal tension force affects more than just this one location on the cable storage reel. As the cable is wound off of the cable reel, the longitudinal tension force is also converted to a transverse force at the cable reel which squeezes the cable. The transverse force which squeezes the cable also migrates down within the layers of the cable on the storage reel because the longitudinal tension force on the cable acts to tighten the cable on the reel. This is due to the movement of the cable storage reel. With the storage reel turning, as a result of gravity, the layers of cable constantly shift position relative to each other. This reduces the friction between layers of cable which in turn allows slippage of the cable, i.e. the longitudinal tension force tightens up the cable on the storage reel. As the transverse squeezing force penetrates deeper within the layers of cable due to slippage, the transverse force is cumulative on the lower layers of cable because the layers of cable above are also exerting this squeezing force on the underlying cable. Since the lower layers of cable on the reel are subjected to an increasingly greater force which squeezes across the diameter of the cable as the cable tightens on the cable storage reel during deployment operations, the cable could rupture, spilling oil or the cable could collapse, damaging the internal components of the cable.
A bottom cable has a similar construction as the streamer cable and undergoes the same process when deployed from a storage reel, the major difference is that a large component of the tension on the bottom cable at the cable storage reel is the weight of the cable between the ship and the bottom. This weight can be substantial especially when the cable is laid at depths in excess of 4000 feet.
The longitudinal tension force on the cable at the storage reel could be reduced by: reducing the towing speed when a streamer cable is deployed; increasing the rate at which the cable storage reel deploys cable; or using sheaves.
Reducing the towing speed is one method now used to deploy streamer cable. However, the longer the cable the more drag exerted on the cable. Consequently, to reduce the tension to an acceptable level at the cable reel, the longer cable must have an even slower towing speed. This can lead to tangling streamer cables if multiple cables are towed.
Increasing the speed on the deployment of the cable from the cable storage reel is another method to compensate for longer cable. This technique could be used with both bottom cable and with streamer cable. However, it is not practical to increase the speed of the storage reel to deploy cable at the same speed as the ship or at the same rate a bottom cable will sink in water for the following reasons: (1) A cable storage reel can be 12 feet or more in diameter and there is a limit on how quickly the cable storage reel can rotate safely in a marine environment which is subject to the wave motion on the ship. (2) A tension force is still necessary on the deployed cable in towing operations to guide the cable to prevent tangling and in bottom cable operations to maintain a straight line over the area to be explored. (3) Since the longitudinal tension is necessary to deploy the cable, increasing the speed of the cable storage reel could reduce the longitudinal tension force at the cable storage reel. However, it is also possible that this will also substantially reduce the friction between layers of cable resulting in less tension force needed to tighten the layers of cable to produce the migrating squeezing force on the lower levels of cable on the storage reel. Consequently, the problem will remain, the transverse squeezing force could build up to a point to damage the cable.
As discussed, since a longitudinal tension is necessary for deployment of the cable, the problem could be resolved if it were possible to isolate the cable storage reel from the longitudinal tension force or at least, substantially reduce the longitudinal tension force at the cable storage reel. Sheaves have traditionally been used to perform this function for cable. However, sheaves have proven not to be effective for reducing tension forces on a marine cable at the cable reel because: (1) The diameter of the marine cable can vary along its length due to the various types of instrumentation packages which can be placed on the cable, (2) connecting couplers which link streamer cable sections and instrumentation packages together to form a streamer cable may have a diameter greater than the streamer cable, and (3) in order for sheaves to reduce the tension applied at the cable reel, it must exert a force transverse to the length of the streamer cable. However, current designs of marine cable can not withstand a transverse force on the outer surface because the outer surface is flexible. The outer surface is supported by spacers and a fluid within the cable. Therefore, sheaves can not exert enough force at a single point on the cable to counter balance the longitudinal tension force upon a marine cable at the cable reel.
One approach to solving this problem is found in U.S. Pat. No. 4,828,223, Cable Handling Apparatus, issued on May 9, 1989 to Russel and Gjestrum. The Cable Handling Apparatus overcomes many of the above mentioned disadvantages of using sheaves by using a series of sheaves which encounter the marine cable at several different locations along its length. The invention can also accommodate varying diameters of the cable with each sheaves in turn absorbing some of the tension caused by the drag force on the cable. However, a device of this nature has a complex pneumatic control system. In addition, it is large and bulky, thereby taking up considerable space aboard a ship.
A simpler approach is desired to reduce the longitudinal tension force at the cable storage reel or to isolate the longitudinal tension force from the cable storage reel during cable deployment operations so that cable deployment can be performed quicker and within the tension levels at the storage reel which will prevent damage to the marine cable.